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Old 6th Feb 2018, 3:36 pm   #1
Al (astral highway)
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Default Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or not?

I have salvaged a junked PC SMPS for the case.

I note from this experience and two that I found in the street previously, that there is what appears to be an (RF) choke in both the +5V and +12V DC power out stage, co-located with big clusters of electrolytics and rectifier diodes. However, I have no way of sussing the inductance.

I am trying to find a suitable core for a home-brew RF choke to use in a power HF circuit (at about 200KHz - rather higher than the SMPS).
The inductance of my home-brew version isn't that critical: approx 220 uH is about right.

(I notice that a commercial one that I bought, supposedly rated 5A, actually saturates below this, at 3A - hence my mission to wind my own. It needs to not saturate at 5A, 30V).

Is the yellow core material indeed suitable for an RF choke of this description?

I only have experience of the grey and green types, kindly supplied by Kalee20 (Peter) in the past.

Thanks, folks.
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Old 6th Feb 2018, 6:22 pm   #2
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

To show this in context, here are two photos. I've clipped a few wires to expose the core, which is what I'm interested in.

I can't recall the exact output current of a computer SMS at 12V and 5V respectively, but I know it's over 12A for one of the DC supplies available.

This choke serves both outputs, and because of the function it appears to serve, I am guessing it doesn't saturate easily. If I am right about its role in the SMPS, the core will easily be suitable for my application.

This kind of unit will of course be very familiar to some forum members with experience maintaining PC's!
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Old 6th Feb 2018, 6:54 pm   #3
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Al,

This looks like a multi-winding choke, for several outputs. As well as being able to use just one core instead of n cores for n outputs, it has other benefits.

But to answer your question - this will be a low-permeability, hard-to-saturate material.

Toroids, when painted, could easily be ferrite (high permeability, makes good transformers, but no good with DC present) or powder (much lower permeability but better for chokes with DC present, indeed anything where precise inductance is necessary).

Down side is that without inductance measuring equipment, it will be hard to determine exactly what.

If you're using your choke at 200kHz, and it is a choke, give it a try!

If it's part of a resonant circuit where Q is important, then you may find losses rather high. Then you really do need to think ferrite, with an air gap.
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Old 6th Feb 2018, 7:35 pm   #4
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Hello Peter,

Thank you! It will be used just as an RF choke and if it’s hard to DC saturate this kind of core, then it seems ideal . I can take an informed guess as to the approx number of turns by looking at the existing windings — especially if someone can tell me through usual switching frequency of these units ?! I have no idea and there are very few clues !

Alternatively, I could build an air-cored choke, although it will be on the large side !

You talk of the several benefits of a multi-winding choke ...what are these ?
I’m guessing that here, in the originals application, it is wound in one direction for one-of the DC supplies (+5V) and in the opposite direction for the other DC supply (+12V) such that one supply partially annuls the magnetisation of the core material imposed by the other?
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Old 6th Feb 2018, 7:58 pm   #5
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Not sure what your 'power HF' circuit is going to be, but be aware that hysteresis/saturation effects in magnetic components can be a nasty source of distortion.

The high-power ends of the linear-amps I designed a few decades ago always used air-cored resonant circuits for this reason.
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Old 6th Feb 2018, 10:10 pm   #6
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Quote:
Originally Posted by G6Tanuki View Post
Not sure what your 'power HF' circuit is going to be, but be aware that hysteresis/saturation effects in magnetic components can be a nasty source of distortion.
Hello, yes, I wouldn’t ever use magnetics in a power LC tuned-circuit; I actually really
enjoy winding precision air-cored inductors, and have used them in tank circuits and experimental VFO’s.

In this circuit , the choke is merely to keep RF out of an external DC power supply to a small class E RF stage.

The signal is generated by a CD4046BE chip and this and the complementary pair driver stage have their own internal power supply with nice military spec tantalum capacitors (using these for the first time, and very impressed) as part of the carefully planned decoupling measures.
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Old 6th Feb 2018, 11:48 pm   #7
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Quote:
Originally Posted by astral highway View Post
You talk of the several benefits of a multi-winding choke ...what are these ?
I’m guessing that here, in the originals application, it is wound in one direction for one-of the DC supplies (+5V) and in the opposite direction for the other DC supply (+12V) such that one supply partially annuls the magnetisation of the core material imposed by the other?
Unfortunately not. There is significant AC voltage across the filter choke, and by transformer action between the windings, if they were in antiphase, the ripple voltages would try to go sky-high on BOTH outputs - that is, if the converter even managed to work...

One benefit is, that with separate chokes, a step change in load current on one output can give rise to a damped oscillation in output voltage, due to the resonance of the choke's inductance and the following filter capacitance. But, if the chokes are all coupled together, the oscillation is transferred to all outputs. Since one of these outputs will be used to regulate the pulse-width of the primary switching transistor, the circuit as a whole can react and clobber the oscillation far more quickly. Response to transient load changes is improved on all outputs to nearly as good as the major, regulated output.

Another benefit is that by subtle tweaks to the turns ratio of the choke, the ripple current can be 'steered' from one output (which might be critical) to another output (which might not matter so much). Or, from a low-voltage high current output, to a higher voltage lower current output, where the rating if the filter capacitor might be able to cope with more than its pro-rata share of ripple.

Down side of all this is that the DC currents in the core all add to the total magnetisation. But the total size necessary for the combined choke is the same as the sum of the sizes of separate components-and a single component is often more cost-effective than separates.
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Old 8th Feb 2018, 7:31 pm   #8
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Quote:
Originally Posted by kalee20 View Post
One benefit is, that.. if the chokes are all coupled together, the oscillation is transferred to all outputs. Since one of these outputs will be used to regulate the pulse-width of the primary switching transistor, the circuit as a whole can react and clobber the oscillation far more quickly. Response to transient load changes is improved on all outputs to nearly as good as the major, regulated output.
Thank you, this is really smart design: saves money and space, and works. I love coming across instances like this.

To round this off, pictured is my home-wound RFC on the yellow former in question. Sure, I can't measure the inductance atm, but I'd say from analysis of quite a few high-current capacity commercial chokes that I've bought over the last few years that it will be comfortably over the non-critical value that I mentioned to start with. So yes, these highly magnetisable yellow cores are ok as formers for high-current RF blocking chokes at HF.

In circuit, it does exactly what it is intended to do, and unlike its commercial predecessor, it doesn't even get slightly warm carrying a heavy DC current.

I just wrapped a couple of thin strips of kapton tape around the outside to keep everything in place.
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Old 8th Feb 2018, 10:17 pm   #9
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Quote:
Originally Posted by astral highway View Post
Sure, I can't measure the inductance atm...
May I recommend the Peak Atlas LCR meter? A few tens of £'s, and it works really well. Obviously has limitations, but it's far better than nothing at all, and you can check your cores!
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Old 8th Feb 2018, 10:44 pm   #10
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

The yellow colour of the core is a sure-fire indication that the core has been dipped or sprayed in yellow paint as far as what that means, it all depends on which manufacturer made it. They've all used yellow and it means quite different materials from different makers. I've seen it on decent RF ferrite materials ('S2' from SEI many years ago) to dust iron (Micrometals inc == Amidon) and even on mumetal dust for low frequencies and strip-wound iron.

Unless you know the provenance, colour is no help.

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Old 8th Feb 2018, 11:36 pm   #11
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Here's a little bit of info for future reference re toroids and line filters.

http://www.vkham.com/Info/ferro/tut_6.html
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Old 9th Feb 2018, 5:33 pm   #12
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

Hi Kalee20! Good recommendation but if I remember correctly, not so good in the uH ranges, or am I confusing it without another? I did used to have one and I wonder why I passed it on to someone else !
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Old 9th Feb 2018, 5:36 pm   #13
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Default Re: Yellow cores in PC SMPS chokes: ferrite or powdered iron - good for RF choke or n

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Originally Posted by Radio Wrangler View Post
The yellow colour of the core is a sure-fire indication that the core has been dipped or sprayed in yellow paint as far as what that means, it all depends on which manufacturer made it. ...

Unless you know the provenance, colour is no help.
Good point, David. Thanks. In this case, I did know the provenance to the extent that I could see what it was being used for in a SMPS - also, the third one of exactly the same colour in the same role in two other SMPS's. So it was an informed, intuitive, let's have a look and see if it works as its role suggests it can.
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